FIG. 9 is a diagram illustrating a configuration of an optical pickup apparatus 110 according to a first related art. FIG. 10 is a top plan view of a first hologram element 111 as a diffraction element, a second hologram element 112 as a diffraction element, and a light-receiving element 113 according to the first related art. FIGS. 11A and 11B are diagrams illustrating relationships between the first and second hologram elements 111 and 112 and the light-receiving region, at which the light diffracted therethrough arrives, according to the first related art. FIG. 11A illustrates a situation in a case of using light with a first wavelength, and FIG. 11B illustrates a situation in a case of using light with a second wavelength.
The optical pickup apparatus 110 according to the first related art includes: a light source 116 that has a light emitting origin 114 for light with the first wavelength and a light emitting origin 115 for light with the second wavelength; the first hologram element 111; the second hologram element 112; and the light-receiving element 113. The first hologram element 111 diffracts a light beam, which is light emitted by the first light emitting origin 114, and a light beam which is light emitted by the light emitting origin 115 for light with the second wavelength. The second hologram element 112 diffracts the light beam (indicated by the chain double-dashed line) emitted by the light emitting origin 115 for light with the second wavelength so as to guide the beam into the light-receiving element 113, and transmits the light beam (indicated by the solid line) emitted by the light emitting origin 114 for light with the first wavelength. The first wavelength is shorter than the second wavelength, and the diffraction efficiency of the diffraction region for light with the first wavelength is lower than the diffraction efficiency of the diffraction region for light with the second wavelength (for example, refer to Patent Literature 1).
FIG. 12 is a diagram illustrating a configuration of an optical pickup apparatus 120 according to a second related art. FIG. 13 is a top plan view illustrating a diffraction element 121 and a light-receiving element 122 of the optical pickup apparatus 120 according to the second related art. FIGS. 14A and 14B are top plan views illustrating a state where light is received using the diffraction element 121 and the light-receiving element 122 according to the second related art.
The emission wavelength of a light emitting origin 123 for light with the first wavelength is a wavelength which is shorter than the emission wavelength of a light emitting origin 124 for light with the second wavelength. The diffraction angle of the light beam (indicated by the solid line), which is emitted by the light emitting origin 123 for light with the first wavelength and is diffracted by the diffraction element 121, is smaller than the diffraction angle of the light beam (indicated by the chain double-dashed line) which is emitted by the light emitting origin 124 for light with the second wavelength. Due to the difference between the diffraction angles, light-receiving regions 125 to be described later are formed in the light-receiving element 122 so as to be arranged in three columns.
The light beam, which is emitted by the light emitting origin 123 for light with the first wavelength, is incident to a light-receiving region 125a of the column corresponding to a small diffraction angle and a light-receiving region 125b of the center column, among the light-receiving regions 125 of the three columns. At this time, the focus error signal is detected by a knife edge method using an output from a predetermined part of the light-receiving region of the light-receiving region 125b of the center column.
The light beam, which is emitted by the light emitting origin 124 for light with the second wavelength, is incident to a light-receiving region 125c of the column corresponding to a large diffraction angle and the light-receiving region 125b of the center column, among the light-receiving regions 125 of the three columns. At this time, similarly to a case of using the light emitting origin 123 for light with the first wavelength, the focus error signal is detected by a knife edge method using an output from a predetermined part of the light-receiving region of the light-receiving region 125b of the center column (for example, refer to Patent Literature 2).
Regarding the light which is originated from the light emitting origin 123 for light with the first wavelength and is reflected by a recording medium and the light which is originated from the light emitting origin 124 for light with the second wavelength and is reflected by the recording medium, the positions of the light beams incident to the diffraction element 121 do not completely coincide with each other. Further, it is difficult to adjust the postures of the light emitting origins 123 and 124 for the light with the first and second wavelengths with respect to the diffraction element 121 to the same positions even in any optical pickup apparatus.    Patent Literature 1: Japanese Unexamined Patent Publication JP-A 2002-92933    Patent Literature 2: Japanese Unexamined Patent Publication JP-A 2007-287278